It is well-known that the process of nuclear fission involves the disintegration of the fissionable fuel material, usually enriched uranium dioxide, into two or more fission products of lower mass number. Among other things, the process also includes a net increase in the number of available free neutrons which are the basis for a self-sustaining reaction. When a reactor has operated over a period of time the fuel assembly with fissionable material must ultimately be replaced due to depletion. Inasmuch as the process of replacement is time consuming and costly, it is desirable to extend the life of a given fuel assembly as long as practically feasible. For that reason, deliberate additions to the reactor fuel of parasitic neutron-capturing elements in calculated small amounts may lead to highly beneficial effects on a thermal reactor. Such neutron-capturing elements are usually designated as "burnable absorbers" if they have a high probability (or cross-section) for absorbing neutrons while producing no new or additional neutrons or changing into new absorbers as a result of neutron absorption. During reactor operation the burnable absorbers are progressively reduced in amount so that there is a compensation made with respect to the concomitant reduction in the fissionable material.
The life of a fuel assembly may be extended by combining an initially larger amount of fissionable material as well as a calculated amount of burnable absorber. During the early stages of operation of such a fuel assembly, excessive neutrons are absorbed by the burnable absorber which undergoes transformation to elements of low neutron cross-section which do not substantially affect the reactivity of the fuel assembly in the latter period of its life when the availability of fissionable material is lower. The burnable absorber compensates for the larger amount of fissionable material during the early life of the fuel assembly, but progressively less absorber captures neutrons during the latter life of the fuel assembly, so that a long life at relatively constant fission level is assured for the fuel assembly. Accordingly, with a fuel assembly containing both fuel and burnable absorber in carefully proportioned quantity, an extended fuel assembly life can be achieved with relatively constant neutron production and reactivity.
The incorporation of burnable absorber in fuel assemblies has been recognized in the nuclear fuel as an effective means of increasing fuel capacity and thereby extending core life. Burnable absorbers are used either uniformly mixed with the fuel (i.e., distributed absorber) or are placed discretely as separate elements in the reactor, as separate burnable absorber rods, so arranged that they burn out or are depleted at about the same rate as the fuel. Thus, the net reactivity of the core is maintained relatively constant over the active life of the core.
Among the various burnable absorbers that have been mixed with fuel as a distributed absorber, gadolinium oxide has been found to be an excellent absorber due to its extremely high thermal absorption cross-section. Enriched uranium dioxide, with a high U-235 isotope content, and gadolinium oxide, as a mixture, has thus previously been used as nuclear fuel pellets.
The use of separate bodies or pellets of a burnable poison in conjunction with nuclear fuel pellets has also been proposed. In U.S. Pat. No. 3,334,019 for example, the use of poison plates containing boron or a boron compound, dysprosium or samarium, cadmium or europium, has been proposed, where these plates are disposed between fuel elements containing fissile material. The purpose of the interspersing of poison plates between the fuel elements is to control the tendency of the reactivity of a reactor to change during its life. Also, in U.S. Pat. No. 3,119,747, a fuel element is described wherein wafers of a burnable poison are disposed on either side of a fuel body, and cylinders of a moderator, such as graphite, are disposed between the wafers and the respective end fixtures for the fuel element.
As discussed above, during operation of the reactor, fissile materials are released from the fuel pellets. These released materials, which include volatile materials, cause a problem of stress corrosion and possible failure of the metallic tubular cladding. This phenomenon is generally described as "pellet-clad interaction" (PCI). The chemical reaction of the metallic tubing with volatile fissile materials such as iodine, cadmium, or other volatile elements, coupled with cladding operating stresses can produce stress corrosion cracking of the metallic cladding or tubing and eventual penetration of the wall of the tube. Attempts have been made to prevent such pellet-clad interaction, such as by coating the inside wall of the tubing with a protective coating, and co-extruding a pure zirconium barrier on the inner portion of the zircaloy tubular wall. Such procedures are objectionable because of the high costs associated therewith.
It is an object of the present invention to provide a nuclear fuel rod that is so constructed as to eliminate or minimize pellet-clad interaction.
It is another object of the present invention to provide a nuclear fuel rod that incorporates the eliminating or minimizing of pellet-clad interaction failures into a burnable poison concept.